The art of manufacturing foamed foodstuff by processing powdered and granular ingredients using a screw extruder is well know. In the prior art the ingredients must be added with a liquid and fully kneaded before the ingredients are inserted in the extruder. Therefore, the prior art necessitated a separate conventional kneader, in addition to an extruder, which is a troublesome manual operation. Further, as the ingredients are slurry-like, it is difficult to heat the ingredients uniformly in the extruder and to obtain homogeneous products. Further, the prior art has the disadvantage that when slowing down the rotation speed of the screw for the purpose of avoiding the foregoing problem, e.g. uniform heating, not only the productive efficiency decreases but also the residence time in the extruder is prolonged, such it is difficult to control the degree of conversion of the starch in the ingredients to the alpha formation thereof. Consequently the ingredients must be extruded as-half cooked from the extruder, and thereafter the ingredients must be baked in the oven for the purpose of foaming.
The object of this invention is to provide a method of manufacturing a foamed foodstuff wherein the above mentioned drawbacks inherent in the usual manufacturing method are eliminated; the prior art operation of kneading the ingredients prior to extrusion and the requirement for a conventional kneader are eliminated; the mixing and heating of the ingredients, addition of a liquid, kneading and heating of the ingredients and the required conversion of the starch to the alpha form thereof are carried out in the same cylinder of a single extruder which operations are successively and automatically performed to achieve a high productive efficiency; and further, the obtained products which are homogenous and of a uniform expansion.